1. Field of the Invention
This invention relates to a method for treating polyarylene sulfide resins and in particular to such a method by which the corrosive property of the resins is effectively reduced or diminished.
2. Prior Art
Polyarylene sulfide resins exhibit excellent heat and chemical resistance properties and high fire-retardant property and, hence, have attracted a good deal of attention as engineering plastics. The resins have been in increasing demand in various fields where the above-mentioned properties are wanted; for example, in the production of electrical and electronic parts, automotive parts, films, sheets and fibers. The resins have also good molding and forming characteristics and high insulation property. In order to meaningfully enjoy these advantageous properties of the polyarylene sulfide resins when employed in practice, it would be required to diminish or substantially eliminate the corrosive property of the resins. For example, where the resins are used as covering or encapsulating material for electronic parts, the material could corrode the electrode, wiring, conductor, leading frame and other metallic elements of the parts in use and often could result in functional trouble of the parts.
As a means to reduce the corrosive property, it has been proposed to chemically treat polyarylene sulfide polymers after polymerization or further purification stage. For example, Japanese Patent Application KOKAI 57-108136 describes a method for treating a polyarylene sulfide polymer which has been polymerized and purified, which comprises washing the polymer in an aqueous slurry containing a salt of metal from Group IA, II or IIIA of the periodic table, an organic acid, ammonia, a basic ammonia compound, or an alkali metal or alkaline earth metal hydroxide. Japanese Patent Application KOKAI 62-36425 describes a treatment method comprising mixing a polyarylene sulfide resin with an inorganic ion-exchanger in the presence of deionized water and/or N-methyl-2-pyrrolidone and heating the mixture. However, these known methods do not appear to be effective to sufficiently reduce the corrosive property of the polyarylene sulfide resins. Therefore, it is necessary to repeatedly conduct the same procedure to lower the corrosive property down to an acceptable level. Thus, the known methods would not be applicable in practice.
It is also known that zinc compounds are added to polyarylene sulfide resins; see, for example Japanese Patent Application KOKAI 1-135866. In this KOKAI specification, a polyarylene sulfide resin is melt-kneaded with zinc carbonate so as to retard the corrosive property and also stabilize the color tone. However, since polyarylene sulfide resins exhibit high melting temperature, the zinc carbonate additive, which decomposes at 300.degree. C. while evolving carbon dioxide gas, tends to produce bubbles in the molded or shaped product and hence to lower the mechanical strength of the product and roughen the surface of product with the bubbles. These present problems in practice.